Pad printing machines comprise an ink cup which is supported in inverted fashion with a sealing end thereof on a cliche that is mounted in reciprocating fashion for transferring ink in a predetermined printing pattern to a printing pad. The ink cup includes an annular surface, which may be an integral part of the cup, or alternatively, a separate ring, that serves as a sliding seal between the ink cup and the cliche and as a doctor blade or "knife" for ensuring that only the engraved portions of the cliche carry ink to the printing pad pick-up site. U.S. Pat. Nos. 4,557,195 and 4,905,594 disclose examples of prior such machines and their disclosures are incorporated herein by this reference.
To ensure quality printing with pad printers, it is important that the annular doctor blade of the ink cup reliably scrape or wipe from the cliche plate all ink that is not within the engraving recesses. Consistently obtaining the clean wiping action has presented problems, particularly when the cliche plates become warped or there are anomalies in the ink cup hold down mechanism. To obtain and maintain a sealing and wiping action, it has been proposed in said U.S. Pat. No. 4,557,195 to provide highly accurate microfinished surfaces on the doctor element and on the cliche and to allow slight swiveling movement of the ink container relative to the contact surface of the cliche. Downward force was applied to a shoulder around the ink container near its lower end by a compression spring for the stated purpose of generating a contact force approximately uniformly distributed over the end face.
The U.S. Pat. No. 4,905,594 proposed a different approach of allowing the doctor knife portion to move relative to the ink cup for adjustment purposes, with few critical dimensions, by providing a flexible connection of the doctor ring to the ink cup, together with a hold down mechanism which applied pressure at only a few predetermined points about the periphery of the ink cup. A commercial machine design of the assignee of said U.S. Pat. No. 4,905,594 utilizes a doctor knife ring made separately from the cup and mounted in the lower end of the cup, with a relatively soft plastic insert between the knife ring and the cup to allow relative adjustment movement between the knife and the cup. A hold-down pressure ring is mounted on pivot pins which extend horizontally transverse to the axis of movement of the cliche and provides three-point force transfer to the cup flange near the knife ring.
While such prior arrangements have met with some success, they require modification of the cup design and its mechanical function. Also, they can result in uneven application of contact pressures to various portions of the knife ring. Further, they permit fore-and-aft rocking tendencies of the knife ring which can vary the contact pressure with the engaged cliche as the cliche is reciprocated rapidly, which can result in undesirable variations of the scraping effect of the knife blade on the cliche.